The present invention relates to polymer chemistry, and, more particularly, to a method of minimizing defects due to shrinkage during polymerization.
While not so limited in scope, the present invention arose in the context of polyacrylamide gel electrophoresis, in which columns of gels are prepared in tubes. During electrophoresis, an ionic sample is located at one end of the column. The ionized components migrate differentially according to charge and bulk under the influence of an axially applied electric field. After a predetermined time, the electric field is removed and the components analyzed according to axial position along the tube.
The columns of gel can be prepared by filling a tube with an aqueous mixture of acrylamide monomer, and then polymerizing the monomer. In the case of acrylamide, as is generally true in polymer chemistry, the polymer is substantially denser than the original pre-polymer, e.g., the monomer, dimer, or oligomer, from which the polymer is formed. Accordingly, significant shrinkage occurs during polymerization.
As a consequence of this shrinkage, the forming gel has a tendency to pull away from the interior walls of the tube. The voids thus formed between the tube and the gel can disturb the uniformity of an applied electrio field, and seriously diminish the resolution of the electrophoresis process Furthermore, the separation of the gel from the tube aggravates a tendency of the gel to migrate out of the tube during electrophoresis.
These problem can be addressed by coating the interior of the tube with a bonding agent which forms covalent bonds between the surface of the tube and the polymer chains While separation and resulting migration are mitigated, the tension introduced by tendency to shrink during polymerization can cause bubble-like voids within the gel itself. These internal voids also distort an applied electrio field and diminish the resolution of the electrophoresis process.
The voids due to shrinkage are not amenable to procedures used to minimize bubbles formed from dissolved gasses. The latter can be minimized by degassing the prepolymer or by conducting polymerization under moderate pressure of one hundred and some odd pounds per square inch (psi). While effective at preventing bubbles formed from dissolved gasses, these methods have negligible impact on voids induced by shrinkage.
The problem of shrinkage-induced voids is not limited to electrophoresis gels. Polymerization is generally accompanied by increases in density, viscosity, and, particularly in the cases of cross-linked polymers, rigidity. Where the polymerizing substance is constrained in some way, e.g., by vessel walls and/or a bonding agent, it can occur that the polymerizing substance cannot respond to continuing increases in density in a fluid or elastic manner. In such cases, the forming polymer structure can rupture haphazardly. This is problematic where it is desired to precisely control the dimensions and uniformity of the resulting polymer.
Thus, what is needed generally is a method of preventing defects induced by shrinkage during polymerization. Specifically, it is desired to reliably produce columns of polyacrylamide gel without voids internal to the gel or between the gel and the interior wall of the confining vessel, or shrinkage of the gel along the axis of the column.